Process and apparatus for treating molten metals



Dec. 1, 1964 R. F. GOTTSCHALK 3,159,478

PROCESS AND APPARATUS FOR TREATING MOLTEN METALS Filed July 28. 1960 FIG. 2

M T SURFACE g INVENTOR.

R0) 1? GOTTSCHALK ha Z.

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United States Patent Ware Filed July 23, 1960, Ser. No. 45,861

1 Claim. (Cl. 7593) This invention relates in general to a method of treating molten metals, such as aluminum and aluminum alloys, to remove gases such as hydrogen and other impurities in the metal prior to casting. More particularly, this invention relates to a flushing cap device used in conjunction with a lance injection-tube for more eflicient removal of said impurities.

Gases such as hydrogen and oxide inclusions in metals, such as found in aluminum, are familiar problems to metal producers. Removal and control of these impurities are particularly bothersome in the melting and casting of light nonferrous metals such'as aluminum and aluminum alloys. Hydrogen presentin primary and secondary ingot products is undesirable because of its resulting effects on the quality of the rolled product and on the metal-gas content during the remelt operations. In essence, then, the problem of containing and controlling gaseous impurities such as hydrogen in aluminum is related to the casting application and the required properties. Included oxides in aluminum products are always objectionable. Such oxides are detected by their eifect on physical properties, surface finish, machining quality, and on drawing operations. As a result, it is essential that aluminum melting operations be designed to minimize the generation of oxides or the possible inclusion of formed oxides, as well as to remove undesirableincluded gases.

Hydrogen removal from aluminum melts by the use of nitrogen, a gas relatively inert to molten aluminum, or other similar gases is generally accepted to be a process involving a combination of agitation, cooling,- and. partial pressure gas diffusion. Such gas flushing techniques make possible a large scavenging gas-metal interface between a gas essentially free of hydrogen and a molten metal with relatively large amounts of dissolved hydrogen. With this technique, the removal of dissolved gases occurs by diffusion from the metal into the bubbles of the relatively but inert dissimilar gas. As long as the hydrogen diffuses freely from the metal, continued flushing over a period of time will lower the concentration of this gas in the molten aluminum. Unfortunately,

when using the inert gas method in the lance injection technique, the surrounding atmosphere also affects the rate of hydrogen removal from the bath. The rate of hydrogen removal is affected because the molten bath tends to reabsorb hydrogen by reacting with atmospheric moisture and absorbing hydrogen from combustion by products.

It is therefore a primary object of this invention to provide an oxygen free atmosphere overlying the molten metal being treated by gaseous injection;

It is another object to provide a blanket of dry nitrogen or other non reacting gas above the molten metal and the injection tube outlet with a chamber or flushing cap disposed about the injection tube to contain the blanket.

3,159,478 Patented Dec. 1, 1964 Ice Still another object is to provide a process for removing hydrogen and other impurities from the molten metal by injecting nitrogen under the metal surface to agitate the metal While simultaneously passing nitrogen through a flushing cap disposed over the agitated area of the bath, to displace all impurities from the artificial atmosphere as they rise from the melt and enter the cap and atmosphere. I

It is another object to provide a moving artificial atmosphere above the agitated area of the bath whereby the entry of rising difiused hydrogen is purged into the outer atmosphere by the artificial created atmosphere.

A feature or this invention resides in the construction of the flushing cap and its relationship with the injection tube, gas supply, and the molten metal itself.

Other objects, features, and advantages of this invention will become more apparent as the following description proceeds taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective'view of an injection tube and flushing cap arrangement.

FIG. 2 is an elevation partly in section of the flushing "cap, injection tube and related conduit of FIG. 1.

Referring again to the drawings in greater detail, numeral 10 represents a simple embodiment of a metal treating apparatus according to the teachings of this invention, namely, a metal or refractory tube 11 with an open end 12 having disposed about its upper end an enclosure or flushing cap 14. A, conduit 13 communicates with the interior of the injection tube 11 and another conduit 15 communicates with the interior of the cap 14. The flushing cap 14 is also provided with an orifice 19, preferably opposite the entrance of the conduit 15 into the cap.

A threaded pipe bushing or reducer 16 is provided for securing the injection tube at one end and the conduit 13 at the other end. (See FIG. 2.) The reducer 16 is secured within a threaded bore 18 in the flushing cap 14. The reducer may be utilized to accommodate different size conduits and injection tubes, as desired depending upon the gas flow to be utilized and the size of the receptacle or ladle in which the'fiushing operation takes place; The conduits 13 and 15 are connected through suitable flowmeters (not shown) to a source or sources (not shown) of non-reactive gaseous material such as nitrogen. Y

The tube 11 may consist of a single material such as graphite, refractory coated steel, silicon carbide, a special cast iron or other satisfactory alloy. The tube 11 may be of a cylindrical or tubular shape, as shown, however, other suitable shapes may be employed. The flushing cap 14 also may consist of a single material such as a refractory coated low carbon steel, graphite, or suitable crucible material. The cap may be bell-shaped as shown. Here again however, other shapes may also be employed.

The injection tube 11 although shown in the drawing as protruding through a center bore in the flushing cap, may enter anywhere in the surface of the cap, or may even be separate and inserted beneath the molten metal with its outlet below the cap. Thus the drawing merely illusftrates a simple preferred embodiment but other arrangements could also be employed. This will become more apparent from the description to follow.

The melting furnace for melting the metal, such asfor aluminum and aluminum alloys, may be a gas, coke, coal or an oilheated type furnace. The details of the furnace structure per se form no part of the present invention and since such furnaces are in common use, further description is deemed unnecessary. Having briefly described the preferred apparatus to be utilized in carrying out the process, the process utilizing this preferred apparatus will now be described.

In carrying out the invention, an injection tube 11 is immersed in the molten melted metal 20 containing the hydrogengas and other impurities. The tube :is preferably immersed into ithe melt from the surface at adepth equal to about-one to one and 'a half times the-diameter of the'fiushingcap .foroptimum results. Depths deeper than this were found to cause a portion of the elements agitated to bypass the:flushing cap when they arose to the surface, which will -be more clearly understood in the ensuing description.

The outer rimot-the flushing [cap 14 is partially immersed intthemolten metal just a-few inches below the surface'of the melt. A zgaseous material, such as nitrogen or argon, is .passed'from a source through conduit .13 into-the injector tube 11 under Esuflicient pressure to exceed the static-head of thetmolten metal and at a rate of flow sufficient to produce a rapid agitation of the moltenmetal. A-gaseous material-from the'same source, or a second source, is passed-through conduit "15 into the flushing cap :14 at .a pressure slightly greater than atmospheric to maintain a dry oxygen free atmosphere overlying the agitated 'metal. The 'flow (rates may be from about five to twenty-five cubic feetper hour for both the subsurface injection-and the gas passing into the cap. .The total gas consumption is determined by the quantity .of metal to the .treated, the dissolved or inclu ded.v gas content inthe molten metaL andthe quantity of included .oxidesthat are to 1be-removed.

The resulting agitation underthe surfaceof thernelt causes the entrapped .gases (such as hydrogen), oxides, and flux materials included in the molten-metal'tomise to the surface within .thes flushing cap. More particularly, .the partial pressure phenomena resulting from the nit-rogengasinjected blow .the melt causes the hydrogen entrapped linthe .melt .to enter 1 the nitrogen bubbles and be carried to the surface. The hydrogen gas in thus diffused from themetal. -Moreover, the :included oxides are dislodged to thuswagg lomerate. and floaton the bath surface under the flushing capiduring the agitation period where they. can latereasily the removed.

The'gas such as nitrogen ,passingintothefiushing cap from the sourceprevents therising. materials:from-.being exposed to the externalatmosphere. .Moreiparticularly, an artificial atmosphere orgaseous. blanketcreatedabove the agitated melLpermits free diffusion .and ,prevents.reabsorption of the hydrogen. I'Moreover, this tgaseous blanket creates a 'hig'h tension area which .purges .the diffused hydrogen into the external atmosphere viaan orifice 19 in the flushing cap. 14.

As the hydrogen diffuses freely, it was .found .that optimum results were-obtained-lby a continued flushing in one location for approximately -fifteen minutes witha total flow of thirty cubic 'feet of nitrogen ,per .hourvfor four hundred. pound heats. Thev hydrogen concentration was lowered substantially in the melt. 'The nitrogenggas consumptionisthus correlatedto the quantities .of metal to be treated, the dissolved or included gas content in the moltenmetal, and the quantity of included particles that are to be removed.

As previously mentioned, for optimum results, the depth of'theinjection tube into the melt should be from one. to one and a half times the diameter of'the flushing cap. This is understandable since 'the deeper the injection tube is immersed in the melt, the wider range upward the risingdiffused bubbles may stray awayfrorn beneaththe flushing cap. Tests have proved these ranges successful to insure-that the diffused. gases and the other particles rise directly within the flushing cap. For example, -a flushing cap having a thirteen inch diameter corresponding to a tube having a depth capacity of thirteen to nineteen inches, will be suflicient to limit the area of the rising materials resulting from the nitrogen passing through the tube to within the confines of the flushing cap. This of course presupposes that the gas pressure exerted through the injection tube does not exceed the prescribed pressure of slightly higher than the static head of the molten metal.

The flushing cap 14 need not overlap the entire molten metal container. In fact, it is preferable that it does not when utilizing rather large metal containers, because movement of the injection tube to different locations within the container is desirable to agitate the melt for.predetermined timed intervals at each location until .the entire molten bath has been agitated. However, this is not to exclude an arrangement particularly where a small container might be utilized, and a flushing cap would preferably actas a closurefor the container. "With this latter arrangement the flushing cap itself need :not be immersed into the molten metal, but could be secured by any air sealing means over, or on,'the outer rim of the molten metal container. Thus, an oxygen free atmosphere would still be maintained.

The present invention provides a reliable process and a portable and simple apparatus for flushing:molten;metal of impurities, which can easily be moved from one .container of molten metal to another with a minimum'amount of effort. For instance, the source of -nitrogen:or argon may be in a pressurized container'or containers *having the flow meters attached to the conduits :and all easily transported on a wheeled carriage. The carriage;may also carry upright supports for supporting the conduits with means adapted to move the conduitsiinhorizontal .and vertical directions to position theattachedinjection tube and flushing cap with respect to the molten :metal at a prescribed location. Other transporting means :such as overhead rails and wheeled carriage :could also be utilized.

There is thus provided an advantageous .-process:for purifying molten metals of undesirable oxides, :hydrogen and other impurities. By the utilization of :the novel apparatus provided, a portable means is assured forcontaining a blanket of gaseous material above'the agitated area which offers a faster treatment of the metalsince there isno reabsorption of hydrogen, gas orother,inclusions. Moreover, with the flushing cap all oxides and other particles that rise and agglomerate from the agitation to one specific area can be contained and easily removed. But of the utmost importance thereis provided a treated metal that is freeof undesirable impuritiesthus of high quality. This high quality means asmooth surface finish, better drawing operations, andan ideal metal for precisioned machinery parts where :defects in the metal would be a handicap to successful. operations.

Various changes and modifications, in additionto those suggested herein and such as will present themselves to those familiar with the art, may be made without departing from the spirit of this invention whosescope .is commensurable with the following claim.

What is claimed as new and is desired to'be secured by Letters Patent of theUnited States is:

In a process for degassifying and cleaninga molten metal containing gaseous hydrogen by injecting an inert gas below the surface of said molten metal, whereby the gaseous impurities and injected inert gas are diffused to a point above the surface of said molten metal,,the improvement in combination therewith which comprises the simultaneous steps of (a) injecting a stream of said inert gas below the surface of said molten metal (b) collecting said injected inert gas anddiffused gaseous impurities in a confined space above the surface of said molten metal, said confined space being directly above the locus of injection of said first inert gas,

(c) introducing a second stream of an inert gas different from said first stream into said confined space above the surface of said molten metal, said introduction being directly above the locus of injection of said first inert gas, and

(d) purging the resulting mixture of diflused gaseous impurities and inert gases into the external atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS Hess et a1 June 19, 1962 

